The present invention relates to warming pressurized gas contained in storage vessels while the gas is being exhausted to lower pressure and, more particularly, using a series of fluidly communicating vessels having heat sources located in between them to warm the gas as it is flowing through the storage system while the system is being vented to lower pressure.
The flight control systems of modern aircraft use a flight control computer to generate command signals after interpreting and analyzing inputs from the pilot's controls, air data sensors and other aircraft systems. The position of aerodynamic control surfaces, the configuration of engine nozzles and inlets, and engine fuel controls are adjusted responsive to the command signals. The foregoing adjustments are usually made using electric or hydraulic actuators. Such computer-controlled systems are commonly referred to as “fly-by-wire” systems.
Fly-by-wire systems offer significant advantages over non-computerized systems. The flight control computer can assist the pilot by continuously monitoring and adjusting the aircraft's control surfaces to compensate for changed flight parameters, for example, changes in airspeed. It can also use the inputs from the pilot's controls together with current aircraft flight conditions to provide optimum performance while ensuring that the aircraft remains within its permissible flight envelope. For example, if the pilot pulls back hard on the control stick, the computer will command the control surfaces to a maximum “g” pull-up for the current airspeed and altitude.
In conjunction with military aircraft, the flight control computer can be integrated with offensive or defensive systems to optimally position the aircraft for weapon deployment, or to maneuver away from threats most effectively. In addition, fly-by-wire systems can be used to augment the stability of aircraft that have compromised their stability to obtain a stealthier shape or increased performance, or have had their stability reduced due to damage. Such stability augmentation may require continuously dithering the control surfaces. In each of these cases the computational capability and rapid reaction rate of the fly-by-wire control system allows the pilot to maintain the aircraft in dynamically stable flight and to safely maneuver it, whereas the numerous sensory inputs and split-second response times would probably overwhelm a human pilot acting without such assistance.
It is essential that modern military aircraft have uninterrupted electrical and hydraulic power to operate their fly by wire control systems, as it can take mere seconds without a correction for such an aircraft to become uncontrollable. It is therefore imperative for such aircraft to have a backup system to supply electrical and hydraulic power almost instantaneously in the event of the failure of the primary power systems. The backup system is designed to provide emergency power for a relatively short period, e.g., from one to ten minutes. It is intended to provide the pilot with the opportunity either to remedy the problem with the primary electrical system, to land the aircraft, or to properly orient the aircraft to enable him and any other occupants to safely eject from the aircraft.
The emergency power system uses a turbine to drive an electrical alternator or generator, and a hydraulic pump. The turbine wheel is rotated by expanding gases produced by combustion of a mixture of fuel and oxygen in a combustor. The combustion must occur reliably at even the highest operating altitude, where the oxygen content of the air is quite low. Accordingly, to ensure the availability of emergency electrical power throughout the flight envelope, the oxygen for the combustor is stored on board in a pressurized vessel containing oxygen, air or oxygen-enriched air.
As the stored gas is exhausted into a lower pressure downstream of the exhaust valve of the pressurized vessel, its temperature decreases as it expands in accordance with the Joule-Thompson effect. Moreover, the temperature of the gas remaining in the vessel also decreases as the result of the polytropic expansion of the contained gas. Due to the foregoing, the total temperature drop in the exhausted gas can be significant if the ratio between the initial stored gas pressure and the final stored gas pressure is large and if discharge occurs quickly. For example, a temperature drop of approximately 100° F. has been observed during a two minute discharge from an initial stored gas pressure of 5000 psi to a final stored gas pressure of 1500 psi.
The cooling of the gas is undesirable for several reasons. The low temperature inside the storage vessel increases the density of the gas therein. This proportionally increases the mass of gas remaining in the vessel when the vessel pressure becomes approximately equal to the downstream exhaust pressure and the gas no longer flows out of the vessel. The mass of unusable gas remaining in the vessel thus increases as the temperature decreases. The necessary quantity of useable gas could nonetheless be stored by simply increasing the number or size of the vessels. However, the weight and the space that would be necessary to store additional vessels of pressurized gas are at a premium.
Moreover, the exhaust valve or downstream flow control valve used to meter and control the exhaust flow from the vessel is intricate and has critical moving components with tight clearances. A lower temperature extreme causes greater contraction of these components, proportionally increasing the overall differential between their dimensions at the high temperature extreme occurring before exhaust, and the low temperature extreme which occurs towards the end of the exhaust interval. This makes the valve's design and manufacture more difficult and expensive.
Furthermore, low temperatures give rise to the possibility that ice will be formed from vapor carried in the gas stored in the vessel, and that this ice will clog the exhaust valve. Extremely low temperatures require the use of special dehumidifying equipment to ensure that the vessels are filled with gas that is extremely dry, so as to prevent the formation of ice. This support equipment, together with the time and labor necessary to properly use it, adds to the overall cost of the emergency power system.
However, regardless of the care and cost involved in the designing and manufacturing exhaust valves to strict tolerances, and attendant to filling the pressurized vessel with gas of extremely low humidity, decreasing the low temperature extreme of the gas inside the vessel inevitably increases the probability that the exhaust valve will bind or suffer a metering error. Decreasing the low temperature extreme thus adversely affects the reliability of a component whose performance, when called upon, will directly affect the likelihood that the pilot will successfully regain primary power, land the aircraft, or safely eject from a properly oriented aircraft.
A decreased low temperature extreme also causes the system's elastomeric seals to become more rigid. This adversely affects their sealing qualities and increases the probability of leakage. As a leak would decrease the mass of pressurized gas available for generating emergency power, the increased probability of leakage occasioned by less elastic seals further degrades the reliability of the backup emergency power system.
In addition, to efficiently burn, the liquid fuel must first atomize, then vaporize. As the temperature of the gas mixing with the atomized fuel decreases, the vaporization of the fuel becomes inhibited. When the gas temperature is sufficiently cold, the fuel will not vaporize and, in an extreme case, may even freeze. Either of the foregoing would prevent or delay the ignition of the fuel, and adversely affect the performance of the turbine.
One solution to the problems outlined herein comprises igniting an incendiary device located inside the vessel to increase the temperature and pressure therein. More particularly, U.S. Pat. No. 4,965,995 and its divisional patent, U.S. Pat. No. 5,070,689, disclose positioning an incendiary device inside the pressurized vessel and a pressure sensor in the outlet of the vessel. When the pressure drops to a level that is insufficient to provide the desired flow rate of oxidant to the combustor, the incendiary device is ignited by the sensor. Alternatively or conjunctively, a temperature sensing probe may be located within the vessel to ignite the incendiary device when the temperature drops to a predetermined level. The ignition of the incendiary device raises the pressure within the vessel as a result of the explosion of the material of the device or from the heating of the oxidant within the vessel, or both.
One drawback to this approach is that it requires storing an incendiary device on board the aircraft, where accidental detonation from any one of several causes could injure personnel, damage the aircraft, or disable the emergency power system. For this reason, the use of incendiary devices onboard aircraft is avoided.
Furthermore, the incendiary device must contain a fuel and a quantity of oxidant such that after all of the fuel is reacted, the oxidant concentration within the vessel remains almost unchanged from its original concentration. In addition, the ignition of the incendiary device may cause the formation of particulate matter, such as carbon soot, as a by-product. Unless this possibility can be categorically disregarded, a filter must be positioned immediately upstream of the outlet to avoid clogging the flow control valve located downstream of the outlet.
Other approaches have heated the air or oxidant downstream of its exhaust from the pressurized vessel, and before its being mixed with the fuel. For example, U.S. Pat. No. 4,979,362 at column 4, lines 6-15, discloses a heat exchanger heating oxidant flowing from a pressurized vessel, then combining the heated oxidant with fuel and introducing the mixture into a combustor. U.S. Pat. Nos. 4,777,793 and 4,934,136, the latter being a division of the former, disclose mixing air coming directly from a high pressure tank with air which has been heated by a heat exchanger, then mixing this heated air with fuel and igniting the foregoing mixture in a combustor.
However, in heating the air or oxidant downstream of the exhaust valve of the pressurized vessel, the foregoing solutions ameliorate only the problem of fuel vaporization being inhibited by mixing the fuel with cold gas. Since the gas contained in the pressurized vessel remains unaffected, heating the downstream gas does not improve the expulsion efficiency for the gas remaining within the vessel. Furthermore, the foregoing approach fails to solve the problems of the hardening of the elastomeric seals and the contraction of the components of the vessel's exhaust valve.
As may be seen from the foregoing, there presently exists a need in the art for an apparatus which warms the gas used in an aircraft's emergency power system, while overcoming the shortcomings, disadvantages and limitations of the prior art. The present invention fulfills this need in the art.